Weighing up the optionsNovember 2006

With the Government pumping money into low carbon technologies, and local authorities demanding 10 percent of energy to be generated by renewables, what options are open to designers? Nigel Anderson investigates.

Anyone wondering whether a shift from fossil fuels to renewable forms of energy is really needed should listen to Samantha Heath, director of the London Sustainability Exchange: "The Thames Barrier was originally designed to close four times a year - it's now closing 19 times a month."

Through the Energy Act, the UK has an established target for 10 percent of electricity to come from renewable sources by 2010, with an aspiration for 20 percent by 2020. Many local authorities, notably the Greater London Authority and the London Borough of Merton require property developments to generate 10 percent of their energy needs from renewable forms of energy.

Sources of financial support

Energy suppliers are offering grants typically in the region of £1500 for wind power or for heat pump installations in order to meet their statutory energy commitments and as a way of retaining customers.

The UK Government offers funding through the Low Carbon Buildings Programme (LCBP). The programme provides grants for microgeneration technologies to householders, community organisations, schools, the public and not-for-profit sector and private businesses.

Of the £30 million fund made available by the DTI, £28.5 million has been allocated to provide grants for micro-generation technologies. The £28.5 million is divided into £10.5 million for Stream 1, aimed at householders and communities, and £18 million for Stream 2, which targets medium and large scale projects.

Under the Stream 1 Householder and Community categories, all microgeneration technologies are covered, with differing maximum grants for each type. Electricity generation, for example, has maximum grants of £15 000 for photovoltaic and £5000 for wind turbines and hydro-electric schemes. Heat generation has been set at £1200 for heat pumps, £600 to £1500 for biomass and £400 for solar water heating. Limits for CHP have yet to be set.

All these grants are subject to overall percentages of cost limits, typically set at 25-30 percent for individual householders and 50 percent for communities.

Similar criteria apply to the Stream 2 grants, up to £100 000 for medium scale new build and retrofit projects and up to £1 million for large scale new build and major refurbishments, all limited to 40-50 percent of total costs.

Launch dates for the sections within each Stream ran from May to September 2006, and there is a four-year window before the spend deadline in March 2010. The DTI is also aiming to publish details on an extra £50 million of grant aid, following consultation with industry.

An interim schedule of accredited installers and approved suppliers is available on the LCBP web site, while a new accreditation scheme is being developed by the Building Research Establishment (BRE).

The technology options

When considering the available options for renewables, a designer should seek to save money for the client, reduce CO2 emissions, and comply with Building Regulations and planning requirements. Other considerations include life-cycle cost, lifecycle CO2 emissions, educational value, public relations and independence from the grid.

"The first step is to visit the site and talk to the client to determine which options are worth looking at," says BSRIA's David Bleicher. "For example, while photovoltaic panels will generate power in diffuse light, they are much more effective in direct sunlight. A shadow on one cell can impair the performance of the whole module, so shading by trees and access for cleaning are important considerations."

Bleicher also recommends that designers determine whether electrical output will be monitored. "If your PV panels aren't performing, the grid will pick up the slack. Unless you're monitoring the PV output, you'll be none the wiser," he says.

Heat pumps

Heat pumps are becoming increasingly viable, particularly ground-source heat pumps. As they generate relatively low-grade heat, heat pumps work extremely well when teamed with underfloor heating, although they are less effective for hot water supply.

This effect can be demonstrated by looking at the coefficient of performance (CoP), which reduces with rises in the temperature difference between the medium used to generate heat and the medium used to conduct heat around a building.

For most types of heat pump, the minimum CoP to satisfy Building Regulations is 2z0 (measured at design conditions), although most ground-source heat pumps can achieve a CoP of 3 or 4.

That said, manufacturers' data on renewable systems can be deceptive, and energy modelling is recommended. Some assessment software can be downloaded for free, for example via RETScreen.

Solar thermal panels

Hot water can be easily generated using flat solar panels or glass vacuum tubes roof-mounted at an angle to achieve an optimum aspect to the sun. Vacuum tubes are typically more expensive, but they are more efficient.

Given the UK climate, systems are generally sized to supply half of the hot water demand. The remainder is usually achieved through the use of fossil fuels.

It is not only important to look at different renewable technologies, but also their conventional counterparts. As energy is lost through storage, circulation and distribution, some solar thermal systems may end up emitting more carbon than a conventional system. Again, free energy assessment software such as www.spf.ch can be very useful.

Biofuels

Biofuel (or biomass) heat generation using wood chips, dried grasses, wood pellets or rapeseed oil can achieve efficiencies in excess of 80 percent. At the time of writing the Building Regulations do not impose minimum efficiencies, unless duel-fuel systems are being specified. This is because wood chips sourced within 25 miles of use are generally considered carbon neutral.

Space and maintenance requirements are higher for biofuels than for competing technologies, largely due to the space needed to store the fuel, and for the larger boiler.

Waste burning is governed by various regualtions and Directives, so fuel derived from waste has to be treated differently.

An example of the use of biomass for heating is at Weobley School in Herefordshire, where the boiler burns wood chips from local wood thinnings. The ash is used as fertiliser in the school's garden. The initiative supports the local economy, reduces CO2 emissions and has an educational value.

However, unlike condensing boilers, biofuel boilers are not yet fit-and-forget. Wood chips and pellets have different burning characteristics and calorific value, which may cause problems for the control of boiler temperature, water flow temperatures, optimum start strategies, and maintenance intervals. In many cases, conventional boilers are needed for standby. This not only raises capital cost and extends payback periods, but increases the complexity of control and the amount of management and maintenance.

Chronic problems with a biofuel boiler may result in greater use of a conventional, standby boiler. In extreme cases the switch may become permanent, as peace of mind may be more important for the end user than lower carbon emissions.

Wind power

Small-scale wind power is readily available through technology transfer from the marine industry. These units normally produce DC power, which can be inverted to the AC grid from larger generators, but dedicated small-scale applications are more common.

For example, Brenzett School in Kent received a grant from Dungeness Power Station to install a small wind turbine. However, the modest output is largely used to teach children about wind power rather than to contribute meaningfully to the school's energy needs.

By definition, wind turbines need to be in windy, prominent positions. Large corporations such as superstores and petrol retailers favour the use of prominent wind turbines to promote their corporate responsibility. This can be genuine, as long as the building's systems do not slip into a massive default to on - typical of internal lighting and external floodlights at petrol stations and supermarkets.